Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because it is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Ethylene polymerizations are frequently carried out in a loop reactor using monomer, liquid diluent and catalyst, one or more optional co-monomer(s), and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer may be extruded and pelletized.
In passing from the reactor to the flash tank the pressure drops and the polymer slurry is partially devolatilized, i.e. the volatile components thereof are removed. Insufficient devolatilization of polymer slurry may ultimately result in polymer end products that contain an undesired amount of diluent and/or unreacted reactants, such as e.g. (co-)monomer(s). Devolatilization is improved with higher temperature of the polymer slurry resulting in lower content of volatiles in the polymer. However, high temperatures in the flash lines induce fouling problems, i.e. sticking of polymer product to the interior walls of the flash lines, and degradation of the polymer particles in the flash lines. On the other hand, in the flash lines where temperatures are too low, devolatilization of the polymer slurry turns out to be insufficient, resulting in polymer end products having unsatisfactory properties and qualities.
U.S. Pat. No. 3,428,619 discloses a method for transferring polymer slurry from a polymerization reactor to a flash tank via a transfer line wherein the liquid diluent is substantially vaporized and the polymer particles are dried during the transfer. The transfer line comprises a plurality of externally heated zones, for instance two or four zones, of varying size. The method comprises controlling the flow rate of the materials flowing through the different zones. The external heating of the zones is carried out at a temperature sufficiently elevated to vaporize the liquid present on the solid particles but which is below the softening temperature of the polymer particles.
In view of the above, there remains a need in the art for providing an improved process for the devolatilization of polymer slurry issued from a polymerization reactor.
The present invention aims to provide an improved process for the devolatilization of polymer slurry produced in a polymerization reactor. More in particular, the present invention aims to provide a process wherein devolatilization of polymer is improved.